In general, it is known that the ferritic stainless steel is a material suitable for applications requiring the oxidation resistance such as the exhaust gas purifying device for automobile, combustion cylinder for the stove and the like. In the recent exhaust gas purifying device for automobile, however, the thickness of the plate used in the device is thinned to reduce the resistance to the exhaust gas flowing and mitigate the loading on the engine. And also, the thickness of the combustion cylinder in the stove is thinned in order to enhance the combustion efficiency, whereby the temperature is increased and the cost is reduced. In any case, the thickness reduction is attempted by these means, so that the service life of the stainless steel is considerably degraded.
For this end, ferritic stainless steels having a greater amount of Al have mainly been proposed, but as the amount of Al in the stainless steel increases, the brittleness of hot rolled steel sheet is promoted to increase the degree of cracking or breaking of the sheet passing in the sheet production and further it is impossible to conduct the production in the usual producing apparatus.
As a technique for overcoming the above problem of the high Al-content ferritic stainless steel, there is proposed a method as disclosed in JP-B-2-58340. In this technique, rare earth elements of Ce, La, Pt, Nd and the like are added in a total amount of up to 0.060 wt %, but if it is intended to manufacture products having particularly a thin thickness, there is caused another problem that the working can not be carried out at a usual hot working temperature.
There is proposed another method as described in JP-B-4-8502 (JP-A-63-45351). This method is a technique developed for overcoming the problem included in the method of JP-B-2-58340. The feature of this technique is rolling work having no cracking is made possible to more improve oxidation resistance by adding lanthanoides other than Ce being a serious factor. However, such a technique is required to separate and remove Ce from the rare earth elements (hereinafter abbreviated as "REM"), so that the cost increases and also there is caused a problem that the oxidation resistance of joint portion in a honeycomb structural body is insufficient.
In order to overcome the above problems of the conventional techniques (JP-B-2-58340 and JP-B-4-8502), there is further proposed a method described in JP-A-3-170642. This method is concerned with foils of ferritic stainless steel having not only excellent oxidation resistance in form of foil even in a high-speed stream of a high-temperature combusted exhaust gas but also developing an effect of maintaining the durability as a carrier for a catalyst and cheap productivity. This technique particularly strengthens the bonding between P and Ce to improve the hot workability by adjusting the amount of P added in accordance with the amount of REM. However, it is frequently caused that the P-compound does not effectively act to the oxidation resistance, and particularly the oxidation resistance in joint portion through soldering, welding or the like is considerably degraded.
In the above conventional techniques, there is a further problem to be solved that ferritic stainless steel having excellent toughness and hot workability is not obtained together with the more improvement of the oxidation resistance.
It is, therefore, an object of the invention to provide ferritic stainless steels capable of overcoming the aforementioned problems.